This invention relates to the joining of composite materials, and, more particularly, to the joining of composite materials by induction heating.
A composite material is a material formed from two or more constituent materials which retain their identities within the composite material. One important type of composite material is the fiber composite material, wherein continuous or discontinuous fibers of one material are embedded within a matrix of another material. Composite materials such as fiberglass have long been known, and have been used in a variety of applications.
In recent years, a number of high-performance composite materials of great interest in aerospace and other demanding applications have been developed. Some materials, such as carbon, can be made very strong and stiff if they are in an elongated fibrous form with a diameter of a few micrometers or less. Such very fine fibers cannot be used directly in structural applications, and instead are incorporated into a matrix which holds the fibers in the proper alignment and protects them from damage. Nonmetallic matrix materials such as thermoplastic or thermosetting resins are widely used in such composite materials. Both the fiber and the matrix can be selected to be quite low in density, with the composite material having a high strength-to-weight ratio. These materials have therefore become the leading candidates for specific structural applications in the next generation of aircraft, to replace aluminum alloys.
The fabrication of a structure using composite materials requires somewhat different procedures than the fabrication of the same structure using metal parts. When metals are used, individual parts are formed by machining, rolling, drilling, and similar procedures, and then joined with fasteners or adhesive. When composite materials are used, parts are prepared by laying up prepreg strips of the composite material or filament winding to form the structure directly. The composite material is then consolidated in an autoclave.
Even when composite materials are used, there must be a method for joining different pieces of composite structure. For example, if a wing of an airplane is to be formed from composite materials, the internal stiffening elements such as the ribs and spars are first prepared and joined together, and then the wing skin is joined to this structural framework. Typically, the length of each stiffener is much greater than the transverse dimension of its interface with the skin. For some of the bonded joints in such a fabricated piece, adhesives are readily applied and provide acceptable performance. However, adhesives cannot be readily applied in other bonded joints. Adhesives may be more brittle and less resistant to loss of properties at elevated temperature than the adherends, compromising their mechanical performance. Moreover, many adhesives require a further autoclave curing treatment, and an autoclave capable of holding the entire wing may not be available. Externally applied fasteners such as rivets are particularly disadvantageous with composite materials because of the high stress concentrations which they introduce into the joint.
An alternative approach that has been used in some bonding applications of thermoplastic-matrix fiber composite materials is co-consolidation, in which the matrix polymer is softened and the two adherends are caused to fuse together at their common interface. In one instance, the two parts are placed inside tooling and inside an autoclave and the entire extent of both parts is heated along with the interface. Application of pressure accomplishes the co-consolidation at the interface. However, the costs for tooling, equipment, and energy are high. Accordingly, another approach is to apply localized heating to cause only the interfacial region between the two composite pieces to become plastic and flowable, force the pieces together in this state, and then remove the heat. If this procedure is performed properly, the two composite pieces are fused into one piece. There is little or no evidence of the original interface, and the final part is a single integral piece. This approach is very promising, because premature failure sources associated with the interface or adhesives are not present.
One promising heating source used to provide localized heating is an induction heater. Such a heater typically includes a generally planar coil of tubing, through which cooling water flows, and to which a high frequency electrical primary alternating voltage is applied. The current in the induction coil causes induced currents to flow in electrically conducting portions of the composite materials, such as carbon or graphite fibers where they are used.
By experience it has been found that the composite structures fabricated by the induction heating process typically have nonuniformities in the form of unbonded regions, or regions in which the original adherends have become deconsolidated because of excessive local heating. These unbonded or deconsolidated regions are difficult or impossible to repair, and can lead to premature failure of the structure. There is therefore a need for an approach to the bonding of composite material pieces that achieves the benefits of induction bonding, but ensures a good quality fabricated part. The present invention fulfills this need, and further provides related advantages.